Change Disqualify Leads values programatically

I have a requirement to change the Disqualify Lead optionset values based on a value on the form. I've tried javascript but haven't been successful due to the control isn't on the form.

I found the following below which if true I'm barking up the wrong tree: 

As CRM standard functionality you cannot get control over the “Disqualify Lead” dialog box and customizing it will be treated as unsupported.

https://community.dynamics.com/crm/f/117/t/116736

If I can't do it with javascript (can I?) is there a way to do it with a plugin?

Thanks for your help.